Aggresive Arab/welsh mare

Whenever you move a horse out of it's comfort zone, you are asking for all kinds of behaviorial issues. You may have rode her at her old barn where everything is the norm for her and she is around horses that she has established relationships with, but now she is in an insecure place having to establish new relationship with both people and horses, and it seems the horse relationship is going good, but it's the people one she needs help with. She is pinning her ears and spinning her butt to tell you she direspects you and that she is the leader and you aren't. It is the same behavior she would give if you put her in a pasture with a horse besides her mother, that she didn't know. Push her away. When you go out to the barn or pasture throw your arms up to move her away. Let her know that she can't approach with that attitude. Then you need to do some basic ground work with her to truley set it in stone. If your not sure where to begin here is a great round pen training site.  http://www.naturalhorsetraining.com/RoundPen3.html
This will help you get started. Good luck.

Take a stick with you!!!  
Try to make her back up - a submissive move. 
Make her do a little work for her bad behavior.  Get her moving until she says "okay, we'll do it your way".  (it might take a while) She will give you both eyes, do away with the pressure (drop your arms) and ask her to come to you. (if she ignores you resume the moving) At first, sometimes just standing there is a breakthrough so you may walk up to her and lead her back to where you are (the center).  Then groom her or just love on her.
Be careful!!!

Yep, its definitely a pecking order respect issue. 
 
Round pen respect training and then move onto a halter/long lead respect work.  Clinton
Anderson has some dvds that are easy to follow and replicate.  He's quite popular right now and someone you know may have some you can borrow.

One of the things people often forget about when dealing with an aggressive or difficult horse is saddle fit. I can't pretend to know you or your horse. I do think it is worth your time to have the saddle fit and horse's back checked.  Arabs in particular are known to have flatter and shorter backs than many others.  When horses are in pain, they often react with pinning the ears, bucking etc. I don't know if you're riding english or western, but I ran across a really great western saddle company that had phenomenal prices. The site is midwestsaddle.com.

I'm not sure she said anything about having a saddle on the horse.
 
If she doesn't do it with her mom, she is just trying to establish dominance if she's doing it with you. I'm with the poster who said to carry a stick, only I'd carry a training whip. When she comes towards you in a threatening manner, keep her back.
 
And I'd thwap her every time she turns her butt to you. Try not to hit her hindquarters, that actually encourages kicking. Try to get her over the withers.